Nitrox on boat with air refill

[scubadada]

This is not rocket science, nor brain surgery

 

[rx7diver]

The NOAA CNS oxygen exposure table is simple and easy to memorize. I recommend that you go ahead and memorize these oxygen exposure limits (along with the associated couple of rules). Then, if you have these oxygen limits and rules memorized, and you have an oxygen analyzer and a set of somebody's (e.g., U.S. Navy, NAUI, DCIEM, PADI, etc.) submersible air tables, you'll always be prepared for planning not only your initial dive, but your repetitive dive as well. Do your calculations using your slate and pencil during your surface interval—if you didn't already do them *before* your initial dive.

Doing EAD calculations and oxygen clock calculations (especially when backing things out to calculate a max NDL EAN repetitive dive) are a bit too tricky for me to do in my head.

EDIT. A calculator isn't necessary. But, I would prefer to have a pencil and slate—at the very least.

IVC,

A simple example can demonstrate that one can easily exceed CNS oxygen limits during a repetitive dive if one is not careful enough about these things: Assume you're diving a single OMS 125, initially filled to 3,500 [sic] psig with NOAA Nitrox II (EAN36). Assume both your initial dive and your repetitive dive(s) are at a depth of 100 fsw (square profiles). Assume you have a RMV of 0.5 cu ft/min. Assume two-hour surface intervals. Finally, assume the boat's crew will top up your cylinder with air *only* to 2,640 psig.

Although I haven't actually done the calculations (though they are easily done), I'm reasonably sure that CNS oxygen limits will be exceeded (or nearly exceeded) here.

Safe Diving,

rx7diver

 

[packrat12]

IVC,

A simple example can demonstrate that one can easily exceed CNS oxygen limits during a repetitive dive if one is not careful enough about these things: Assume you're diving a single OMS 125, initially filled to 3,500 [sic] psig with NOAA Nitrox II (EAN36). Assume both your initial dive and your repetitive dive(s) are at a depth of 100 fsw (square profiles). Assume you have a RMV of 0.5 cu ft/min. Assume two-hour surface intervals. Finally, assume the boat's crew will top up your cylinder with air *only* to 2,640 psig.

Although I haven't actually done the calculations (though they are easily done), I'm reasonably sure that CNS oxygen limits will be exceeded (or nearly exceeded) here.

Safe Diving,

rx7diver

Next time you really may want to do the runs..

Assuming Recreational.
Dive 1: NDL: 31 Minutes. 67 Cf used. 10 cf to be filled. final mix: 34.8, CNS: 23%. OTU: 55
Dive 2: NDL: 21, 46 cf used, CNS: 23.6, OTU: 37 total otu: 92

etc.

Even with a large tank on an abnormal square profile, CNS is very difficult to exceed on a rec dive day. OTU is still 1/3 max. Sorry, on a rec dive I just do not track my cns & otu with great concern.

 

[geoff w]

IVC,

A simple example can demonstrate that one can easily exceed CNS oxygen limits during a repetitive dive if one is not careful enough about these things: Assume you're diving a single OMS 125, initially filled to 3,500 [sic] psig with NOAA Nitrox II (EAN36). Assume both your initial dive and your repetitive dive(s) are at a depth of 100 fsw (square profiles). Assume you have a RMV of 0.5 cu ft/min. Assume two-hour surface intervals. Finally, assume the boat's crew will top up your cylinder with air *only* to 2,640 psig.

Although I haven't actually done the calculations (though they are easily done), I'm reasonably sure that CNS oxygen limits will be exceeded (or nearly exceeded) here.

Safe Diving,

rx7diver

It's even easier to exceed MOD on a single dive with this example. MOD for EAN36 at 1.4 ppO2 is 95 ft

 

[Iralub]

Why not just let out the remaining nitrox before filling with air? Problem solved.

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[Freewillow]

@ RX7diver, you must be joking
@iralub: why empty the bottle. No need.

Already 6 pages on a non existing problem. Some guys are wonderful

 

[DogDiver]

He already said he didn't have an analyzer at the time in the first post. Most recreational divers I've encountered don't carry one on the boat. My guess is they don't have one at home either. I suspect it's more common for a nitrox diver to NOT own an analyzer themselves.

This is a problematic issue with inexperienced divers diving a gas other then air. I haven't done a dive on air in over 15 years. Back then nitrox was a Voodoo Gas. According to the experts back then we would all die diving that stuff. [emoji41]


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[Centrals]

IVC,

A simple example can demonstrate that one can easily exceed CNS oxygen limits during a repetitive dive if one is not careful enough about these things: Assume you're diving a single OMS 125, initially filled to 3,500 [sic] psig with NOAA Nitrox II (EAN36). Assume both your initial dive and your repetitive dive(s) are at a depth of 100 fsw (square profiles). Assume you have a RMV of 0.5 cu ft/min. Assume two-hour surface intervals. Finally, assume the boat's crew will top up your cylinder with air *only* to 2,640 psig.

Although I haven't actually done the calculations (though they are easily done), I'm reasonably sure that CNS oxygen limits will be exceeded (or nearly exceeded) here.

Safe Diving,

rx7diver

Could you please do the calculation and then show us the result?
A quick plot on Deco Planner:
First dive: 33m for 30mins with EAN36, CNS is 32%
Second dive(1hr SI): 33m for 30mins with EAN36, CNS is 65%. Incidentally, this is a deco dive!

 

[kelemvor]

This is a problematic issue with inexperienced divers diving a gas other then air. I haven't done a dive on air in over 15 years. Back then nitrox was a Voodoo Gas. According to the experts back then we would all die diving that stuff. [emoji41]

The voodoo gas days must have been something. I've only been diving a few years and at this point nitrox is pretty standard.

 

[rx7diver]

Although I haven't actually done the calculations (though they are easily done), I'm reasonably sure that CNS oxygen limits will be exceeded (or nearly exceeded) here.

Here is my thinking:

1. First of all, for these *recreational* dives, I would be completely comfortable using a P02 of 1.5 ata. So, I would be completely comfortable using NOAA Nitrox II at 100 fsw. (Moreover, this is completely consistent with what was taught in the IANTD Nitrox course I took in 1993 or so.)

2. For *planning* these 100 fsw recreational dives, I would compute using a max depth of 110 fsw (rather than 100 fsw), for which EAN36 has a corresponding PO2 of 1.6 ata, which has a NOAA CNS oxygen limit of 45 min.

Keep in mind, I am assuming you have only someone's (US Navy in my case) standard air tables, an oxygen analyzer, and a pencil and slate (and, maybe, a calculator).

Freewillow, Centrals, et al, does this additional detail help? If I can find some extra free time, I'll actually run through the hand calculations. It will be a fun exercise, and, besides, I'll get a chance to use the new iPhone HP 15C calculator app (reverse Polish notation) I recently purchased from Hewlett Packard!

Safe Diving,

rx7diver